This invention relates to a process for improving the color of trimethylolpropane (TMP) by extracting color-causing impurities generated in the reaction to produce TMP. This improved color TMP is characterized by a reduced acid wash color and phthalic anhydride color--color measurement tests commonly used in the industry for assessing product quality and suitability for certain color sensitive end-uses. This acid wash color and phthalic anhydride color are collectively referred to as reacted color.
Trimethylolpropane is produced by the reaction of n-butyraldelyde and formaldehyde in the presence of a strong base, such as sodium hydroxide. A formate salt is produced as a co-product (sodium formate if sodium hydroxide is used) with the TMP. After neutralization of the excess base, the TMP is recovered from the aqueous solution containing the formate salt and neutralized base generally by solvent extraction, e.g. using solvents such as ethyl acetate, isobutanol, butyl acetate, and the like. The TMP is then separated from the organic solvent phase, for example by distillation. Another method for the removal of the TMP from the initial reaction mixture, separating the TMP from the formate salt and neutralized base aqueous solution, is to use two solvents, one solvent in which the TMP is very soluble, e.g. isobutanol, and a second solvent, in which the formate salt and neutralized base are insoluble, in an amount sufficient to separate the aqueous phase from the first solvent, but not sufficient to separate out the TMP. The organic phase with the extracted TMP and other organic compounds is separated from the aqueous phase by decantation and the first solvent removed by distillation. Typically the second solvent is a non-polar solvent, e.g.: xylene, with a boiling point greater than the first solvent. The TMP/second organic solvent mixture is allowed to settle and cool and the separated TMP is removed by decantation. An example of this separation is published as U.S. Pat. No. 3,956,406 to Palmer et al. The recovered crude TMP is then purified by conventional distillation under vacuum to yield a finished TMP product. These processes generally produce a TMP product which has an acid-wash color of about 5-10 Gardner units or phthalic anhydride color of about 100-300 APHA units. Other TMP processes have produced product having an acid-wash color of about 5-6 Gardner units. However, for many applications, e.g. in producing TMP acrylate esters for radiation-cured coatings, it is desirable to obtain product having an acid-wash color of 3 or less Gardner units and phthalic anhydride color of less than 100 APHA units.